It brings its own vessel and heats flowing fluid.

An immersion heater sits in somebody else's tank.

This is piped inline into a system.

Inlet and outlet are part of the unit.

It suits continuous flow rather than batch heating.

Both use elements; the application differs entirely.

It decides whether the fluid survives contact.

Water carries heat away from the sheath readily.

Oils and heat transfer fluids do not.

Exceeding their limit cokes fluid onto the element.

Coke insulates it and the element then fails.

Specify against the fluid, not the kilowatts.

It insulates the element from the fluid.

Heat cannot leave the sheath as designed.

Element temperature rises further.

That accelerates more coking.

Failure follows quickly once it starts.

The deposit is visible on a withdrawn bundle.

Proof of flow before the elements fire.

An element without flow overheats in moments.

A high-limit thermostat independent of control.

Both are standard, not optional extras.

Test them at commissioning.

Most failures trace back to one of them.

By fluid chemistry and temperature together.

Steel suits many oils.

Stainless suits water and process fluids.

Corrosive duties need higher alloys.

Chlorides are the case to check carefully.

State the fluid and its additives.

Whether the element bundle can be withdrawn.

Clearance behind the unit is required.

That clearance is easily lost to other plant.

A bundle that cannot be pulled means full replacement.

Ask about it before fixing the position.

Mark the withdrawal zone at installation.

The fluid, precisely, including additives.

Flow rate and required temperature rise.

Inlet temperature and system pressure.

Whether flow is continuous or intermittent.

Hazardous area classification if applicable.

A kilowatt figure alone is not a specification.